The present invention relates to a direct injection engine and a combustion method for the engine, and particularly to a direct injection engine for direct injection of fuel into the combustion chamber forming a tumble air flow and the combustion method for this engine.
The direct injection engine of this type according to the prior art is disclosed in U.S. Pat. No. 5,878,712 and Japanese Application Patents Laid-Open Publication Nos.
Hei 10-110660, Hei 10-110662, Hei 10-30441, Hei 10-8967, Hei 6-81656, Hei 9-317475 and Hei 10-169447. In these direct injection engines of prior art, there is a difference of density in the fuel spray itself injected from the fuel injector into the combustion chamber, and this makes it difficult to collect fuel around the spark plug. Ignition performance is reduced when so-called lean burning method (lean or extra-lean burning) is used, wherein the minimum amount of fuel is used for burning. In this case, stratified charge combustion range cannot be expanded to cover a high speed traveling of 80 Km/hour or more or a high engine speed range of 2400 rpm.
In other words, there is a deviation of density in the fuel spray itself of the fuel injector. When the portion of smaller density is injected toward the spark plug, it becomes difficult to perform so-called stratified charge combustion where lean fuel is stratified and ignited; thus, stratified charge combustion is disabled.
To put it more specifically, when stratified charge combustion by tumble air flow is carried out in the light load/low speed range using the direct injection engine of prior art, the piston speed is increased if tumble air flow is produced in the combustion chamber in a light load/low speed range or a heavy load and high speed range (traveling at 120 km/h at an engine speed of 3200 rpm). Then it becomes difficult to ensure fuel evaporation time. Therefore, this makes it necessary to advance fuel injection time, but if the fuel injection time is advanced, spraying will be started when the pressure in the combustion chamber is still low. This leads to increased fuel spraying angle with the result that it becomes increasingly easier to agitate spray fuel with tumble air flow.
Furthermore, fuel may be deposited on the inner surface of the cylinder head, the inner surface of the combustion chamber or piston.
The direct injection engine of the prior art is designed to provide the step of uniform combustion wherein intake stroke injection is performed in the heavy load and high speed range and the generation of tumble air flow is discontinued, thereby allowing fuel to be diffused throughout the entire combustion chamber.
In this case, however, sprayed fuel is deposited on the upper surface of the piston or internal wall surface of the combustion chamber, and mixture with air is hindered, with the result that combustion does not take place due to delayed evaporation of the deposited spray fuel, and fuel is discharged from the engine together with exhaust gas. This tendency is conspicuous in this case.
This results in an increased amount of the unburnt hydrocarbon (THC) and reduced engine performance and fuel economy.
The object of the present invention to provide a direct injection engine characterized by improved engine ignition in a light load/low speed range as in idling operation range and by stratified charge combustion ensured in the operation range from a low to high speed ranges, thereby reducing discharge of gas containing hazardous component such as THC and improving fuel economy.
To achieve the above object, the present invention is characterized by following configuration: {circle around (1)} Fuel spray injected from fuel injector presents an annular form with uniform density when viewed from the surface perpendicular to the spray axis line.
To put it more specifically, a valve body mounted on the valve seat of the fuel injection nozzle and a fuel swirling element having a through-hole for guiding this valve body are provided on the nozzle of the fuel injector. This fuel swirling elements are provided with five or more swirling grooves which are arranged in the radial direction at specified intervals. These swirling grooves are formed at positions offset by a specified amount in parallel with a virtual radial line passing through the centerline of the fuel injection nozzle.
To put it more specifically, the valve body of the fuel injector is formed in a spherical shape, and the diameter D1 of this valve body and the diameter D2 of the through-hole of fuel swirling element has the relation of D2≈1.005D1, and the diameter D3 of the fuel injection nozzle and length L has the relation of L/D3 greater than 1.
{circle around (2)} The following describes another way of achieving the above object in the present invention: The fuel spray injected from the fuel injector exhibits a high/low density when viewed from the surface perpendicular to the spray axis line. The fuel injector is mounted on the combustion chamber so that the denser portion is placed face-to-face with the spark plug.
To put it more specifically, an air flow is formed to move from the fuel injector arranged on the side of the combustion chamber toward the spark plug arranged on the top of the combustion chamber. Just before ignition timing, fuel is sprayed from the fuel injector, and fuel spray is carried by this air flow to reach the spark plug exactly at the ignition timing of spark plug (hereinafter referred to as xe2x80x9ctumble guide methodxe2x80x9d). At the same time, fuel spray injected from the fuel injector presents an annular form with uniform density when viewed from the surface perpendicular to the spray axis line, where the deviation "sgr" of the fuel density of this annular fuel spray in the radial direction is smaller than 15%.
To put it more specifically, an air flow is formed which moves from the fuel injector installed on the side of the combustion chamber toward the spark plug arranged on the top of the combustion chamber, and fuel is injected from the fuel injector slightly before ignition timing. Fuel spray is carried by this air flow to reach the spark plug exactly at the ignition timing of spark plug (hereinafter referred to as xe2x80x9ctumble guide methodxe2x80x9d). The fuel spray injected from the fuel injector exhibits a high/low density when viewed from the surface perpendicular to the spray axis line. The fuel injector is mounted on the combustion chamber so that the denser portion is placed face-to-face with the spark plug. The first invention configured as described above makes it possible to ensure that the combustion performance of the direct injection engine is not affected by fuel injector installation position.
The second invention allows concentrated fuel to be supplied to the spark plug, thereby improving the performance of the direct injection engine.
In the specific invention, spray fuel is carried by the tumble air flow and is sent to the spark plug mounted on the cylinder head (tumble guide method). This reduces the spray fuel feed distance and the amount of spray fuel deposited on the top surface of the piston or inner wall of the cylinder, and improves the density of the spray fuel close to the spark plug, thereby ensuring improved ignition by the spark plug.
As a result, this reduces the amount of THC in exhaust gas, and improves the purification rate and fuel economy at the same time.